Btrfs devel - Dec 2011 - avoid redundant block group free-space checks

It was pointed out to me that the test for enough free space in a block
group was wrong in that it would skip a block group that had most of its
free space reserved by a cluster.

I offer two mutually exclusive, (so far) very lightly tested patches to
address this problem.

One moves the test to the middle of the clustered allocation logic,
between the release of the cluster and the attempt to create a new
cluster, with some ugliness due to more indentation, locking operations
and testing.

The other, that I like better but haven''t given any significant amount
of testing yet, only performs the test when we fall back to unclustered
allocation, relying on btrfs_find_space_cluster to test for enough free
space early (it does); it also arranges for the cluster in the current
block group to be released before we try unclustered allocation.







-- 
Alexandre Oliva, freedom fighter    http://FSFLA.org/~lxoliva/
You must be the change you wish to see in the world. -- Gandhi
Be Free! -- http://FSFLA.org/   FSF Latin America board member
Free Software Evangelist      Red Hat Brazil Compiler Engineer

2011/12/12 Alexandre Oliva
<oliva@lsd.ic.unicamp.br>:
> It was pointed out to me that the test for enough free space in a block
> group was wrong in that it would skip a block group that had most of its
> free space reserved by a cluster.
>
> I offer two mutually exclusive, (so far) very lightly tested patches to
> address this problem.
>
> One moves the test to the middle of the clustered allocation logic,
> between the release of the cluster and the attempt to create a new
> cluster, with some ugliness due to more indentation, locking operations
> and testing.
>
> The other, that I like better but haven''t given any significant
amount
> of testing yet, only performs the test when we fall back to unclustered
> allocation, relying on btrfs_find_space_cluster to test for enough free
> space early (it does); it also arranges for the cluster in the current
> block group to be released before we try unclustered allocation.
I''ve chosen to try the second patch in our ceph environment. It seems
that btrfs_find_space_cluster() isn''t called any longer.
find_free_extent() is much faster now.

(I think that the write-io numbers are still to high, though.)

Thanks,
Christian
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